We won't change this now but this is for next time:

I believe you're doing too much here. A mock should be dumb.

You need:
* a method to render some messages, for your test; this one does not need to be in the mock object
* a method in the mock object, but in the test you only need to know if it was called, so you don't need any other behavior.

Maybe the underlying problem here is that MessageManager should be in its own file ans not in sms.js with the other objects ! :-)